The present specification relates generally to the field of integrated circuits and to methods of manufacturing integrated circuits. More particularly, the present specification relates to a tri-tone mask process for dense and isolated patterns.
Semiconductor devices or integrated circuits (ICs) can include millions of devices, such as, transistors. Ultra-large scale integrated (ULSI) circuits can include complementary metal oxide semiconductor (CMOS) field effect transistors (FET). Despite the ability of conventional systems and processes to put millions of devices on an IC, there is still a need to decrease the size of IC device features, and, thus, increase the number of devices on an IC.
One limitation to the smallness of IC critical dimensions is conventional lithography. In general, projection lithography refers to processes for pattern transfer between various media. According to conventional projection lithography, a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film or coating, the photoresist. An exposing source of radiation (such as light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask, for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive image of the subject pattern.
Exposure of the coating through a photomask or reticle causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) or deprotected areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation.
One alternative to projection lithography is EUV lithography. EUV lithography reduces feature size of circuit elements by lithographically imaging them with radiation of a shorter wavelength. xe2x80x9cLongxe2x80x9d or xe2x80x9csoftxe2x80x9d x-rays (a.k.a, extreme ultraviolet (EUV)), wavelength range of lambda=50 to 700 angstroms are used in an effort to achieve smaller desired feature sizes.
In EUV lithography, EUV radiation can be projected onto a resonant-reflective reticle. The resonant-reflective reticle reflects a substantial portion of the EUV radiation which carries an IC pattern formed on the reticle to an all resonant-reflective imaging system (e.g., series of high precision mirrors). A demagnified image of the reticle pattern is projected onto a resist coated wafer. The entire reticle pattern is exposed onto the wafer by synchronously scanning the mask and the wafer (i.e., a step-and-scan exposure).
Although EUV lithography provides substantial advantages with respect to achieving high resolution patterning, errors may still result from the EUV lithography process. For instance, the reflective reticle employed in the EUV lithographic process is not completely reflective and consequently will absorb some of the EUV radiation. The absorbed EUV radiation results in heating of the reticle. As the reticle increases in temperature, mechanical distortion of the reticle may result due to thermal expansion of the reticle.
Both conventional projection and EUV lithographic processes are limited in their ability to print small features, such as, contacts, trenches, polysilicon lines or gate structures. As such, the critical dimensions of IC device features, and, thus, IC devices, are limited in how small they can be.
The ability to reduce the size of structures, such as, shorter IC gate lengths depends, in part, on the wavelength of light used to expose the photoresist. In conventional fabrication processes, optical devices expose the photoresist using light having a wavelength of 248 nm (nanometers), but conventional processes have also used the 193 nm wavelength. Further, next generation lithographic technologies may progress toward a radiation having a wavelength of 157 nm and even shorter wavelengths, such as those used in EUV lithography (e.g., 13 nm).
Phase-shifting mask technology has been used to improve the resolution and depth of focus of the photolithographic process. Phase-shifting mask technology refers to a photolithographic mask which selectively alters the phase of the light passing through certain areas of the mask in order to take advantage of destructive interference to improve resolution and depth of focus. For example, in a simple case, each aperture in the phase-shifting mask transmits light 180 degrees out of phase from light passing through adjacent apertures. This 180 degree phase difference causes any light overlapping from two adjacent apertures to interfere destructively, thereby reducing any exposure in the center xe2x80x9cdarkxe2x80x9d comprising an opaque material, such as chrome.
An exemplary phase-shifting mask 10 is illustrated in FIG. 1. Phase-shifting mask 10 includes a transparent layer 12 and an opaque layer 14. Opaque layer 14 provides a printed circuit pattern to selectively block the transmission of light from transparent layer 12 to a layer of resist on a semiconductor wafer. Transparent layer 12 includes trenches 16 which are etched a predetermined depth into transparent layer 12. The light transmitted through transparent layer 12 at trenches 16 is phase-shifted 180 degrees from the transmission of light through other portions of phase-shifting mask, such as portions 18. As the light travels between phase-shifting mask 10 and the resist layer of a semiconductor wafer below (not shown), the light scattered from phase-shifting mask 10 at trenches 16 interferes destructively with the light transmitted through phase-shifting mask 10 at portions 18, to provide improved resolution and depth of focus.
As mentioned, various different wavelengths of light are used in different photolithographic processes. The optimal wavelength of light is based on many factors, such as the composition of the resist, the desired critical dimension (CD) of the integrated circuit, etc. Often, the optimal wavelength of light must be determined by performing a lithography test with photolithographic equipment having different wavelengths. When a phase-shifting mask technique is utilized, two different phase-shifting masks must be fabricated, each mask having trenches 16 suitable for phase-shifting light of the desired wavelength. The fabrication of phase-shifting masks is costly. Further, comparison of the effect of the two different wavelengths printing processes is difficult and requires complex software processing to provide a suitable display.
In conventional systems, application of a high transmittance attenuated phase-shifting mask (PSM) makes it difficult to improve the photomargin, or depth of focus, at dense and isolated pitches at the same time. For example, if a high coherence level ("sgr") or off-axis illumination is used, it is possible to obtain a large depth of focus at a dense pitch. A high coherence level can be at a pupil fill factor (PFF) of 0.2 or less. However, the depth of focus is small at isolated pitches using high coherence levels ("sgr"). Pitch is commonly known as the distance between adjacent features or structures. Even using shorter lithographic wavelengths and various resolution enhancement techniques, the pitch is usually constrained to a dimension approximately equal to the lithographic wavelength. Furthermore, despite using optical proximity correction (OPC), the depth of focus for isolated pitches cannot be improved. If a low coherence level ("sgr") is used, a large depth of focus can be obtained at isolated pitches. However, the depth of focus is very small or virtually nothing at dense pitches using low "sgr". Furthermore, use of low coherence value ("sgr") also suffers from side lobe printing. The low coherence level ("sgr") can be at a pupil fill factor (PFF) of 0.80 or more.
Thus, there is a need for an improved phase-shifting mask. Further, there is a need for an improved depth of focus, or photo margin, at dense and isolated pitches in a phase shifting mask. Thus, there is a need to improve the photo process margin, or depth of focus, at dense pitches using sidelobe printing.
An exemplary embodiment relates to a method of integrated circuit fabrication involving phase shifting materials. This method can include providing a layer of chrome providing a layer of phase shifting material over the layer of chrome; providing open spaces in the layer of chrome and layer of phase shifting material according to a pattern; removing selected open spaces proximate other open spaces; and transferring the pattern of spaces to the integrated circuit wafer. The portion of the pattern removed by the removing step is transferred to the integrated circuit wafer by side lobe printing.
Another exemplary embodiment relates to a method of improving the photo margin at dense and isolated pitches at the same time in integrated circuit fabrication. This method can include providing a phase shifting mask including a phase shifting material and areas of densely patterned features; and removing selected patterned features in the areas of densely patterned features on the phase shifting mask.
Another exemplary embodiment relates to a method of improving the photo process margin using side lobe printing. This method can include patterning a mask; removing portions of the pattern by deleting open spaces in densely patterned areas of the pattern; and transferring the pattern from the mask to the integrated circuit wafer. Side lobe printing results in a transfer of removed portions of the pattern to the integrated circuit wafer.
Other principle features and advantages of the present invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.